1. Field of the Invention
The present invention relates to a cathode which is employed for the electrochemical reduction of organic compounds, to processes for the preparation thereof and to the use thereof for the electrochemical reduction of organic compounds.
2. The Prior Art
The electrochemical reduction of organic compounds is an important method for preparing a large number of products. The cathode frequently used in this case is lead, which is distinguished by a high hydrogen overvoltage.
Reductions are carried out on lead plates for example of disulfide compounds (A. Aldaz et al. in Electrochemical Processing Technologies, 1997, Miami, Fla.), to give the corresponding thiols. On the industrial scale, cysteine hydrochloride is obtained by reduction of cystine on lead cathodes (review: T. R. Ralph et al., J. Electroanal. Chem. 1994, 375, 17). High chemical yields are obtained in this case with current densities of up to 700 A/m2. Due to the competing evolution of hydrogen, the current yields are limited to about 46%. Various authors describe progressive passivation of the lead surface in this reaction (C. Daobao in Jingxi Huagong 1998, 15, pp. 258 et seq. and pp. 297 et seq., M. Li ibid., p. 294) even at current densities of 200-500 A/m2.
Further examples of the use of lead cathodes are the reduction of aldehydes, ketones, carboxylic acids and carboxylic esters to alcohols, the reduction of heterocycles and dehalogenations (review: M. M. Baizer in Organic Electrochemistry, published by Marcel Dekker, New York 1991, pp. 362 et seq., X. Nishiguchi et al., Electroorg. Synth. 1991, p. 331). Numerous processes are also carried out on an industrial scale with lead cathodes (review: xe2x80x9cIndustrial Electrochemistryxe2x80x9d by Pletcher and Walsh, Chapman and Hall, London 1989, pp. 313-319).
Because of the low mechanical stability of lead associated with the high intrinsic weight and the difficulty of making contact, lead-plated support cathodes, for example made of copper or titanium, are preferred for industrial applications in which large electrode areas are required (M. M. Baizer in Organic Electrochemistry, published by Marcel Dekker, New York 1991, p. 274). However, appropriate coating processes are costly and complicated. Flat geometries are imperative in this case, so that the coating adheres well to the support. It is also known that lead surfaces are easily deactivated (EP 0 931 856), leading to a decrease in the current yields and the area-time outputs. Corrosion of lead electrodes has also been described. To regenerate the cathode surface it is necessary each time to dismantle the electrolysis cell completely. The resulting long nonproductive periods greatly restrict the commercial usefulness.
DE patent 1 024 518 discloses that cystine can be reduced to cysteine with a 100% current yield on tin cathodes with a current density of 500 A/m2. Our own experiments show that the current yields fall below 30% on increasing the current densities up to 2000 A/m2 on tin because, under these conditions, the evolution of hydrogen starts considerably earlier. In addition, the current yield decreases further in subsequent experiments.
It is an object of the present invention to provide a cathode which has a high hydrogen overvoltage, can easily be produced and regenerated, and is suitable for the reduction of organic compounds on the industrial scale.
The above object is achieved according to the present invention by a cathode comprising an electrically conducting support with a coating of electrochemically deposited lead with a density between 0.001 and 2 g/cm3.
This lead coating or layer has a loose structure with a density which is greatly reduced compared with solid lead and is in the range of density between 0.001 and 2 g/cm3, preferably between 0.01 and 1 g/cm3. The increase in the surface area provided by the reduced density has an advantageous effect on the area-time output of the chemical processes.
The support or the cathode of the invention comprises an electrically conducting material, preferably a metal, excepting an alkali metal or alkaline earth metal, or a metal alloy or graphite.
The electrically conducting support particularly preferably comprises a metal or a metal alloy selected from the group consisting of copper, nickel, tin, zinc, titanium, iron, steel, stainless steel, cadmium and lead.
Elements which may also be present as constituents of the alloys are selected from the group consisting of tungsten, chromium, cobalt, molybdenum, manganese, bismuth, aluminium, mercury, zirconium, vanadium, silicon, boron, niobium, tantalum, antimony, phosphorus and carbon.
The term metal hereinafter also encompasses metal alloys as discussed above.
A support of this type can be coated with one or more metals mentioned above. Processes suitable for the coating are all those known to the person skilled in the art, such as, for example, electroplating or sputtering.
The shape of the electrically conducting support is not critical. It is preferably selected from the large number of known and available electrode geometries, for example plate, lattice, foam, disk, tube, perforated plate, rod etc.
The lead coating or layer deposited on the electrically conducting support is distinguished by having a density which is reduced compared with solid lead and is in the range from 0.001 to 2 g/cm2, preferably between 0.01 and 1 g/cm3.
The invention further relates to a process for producing a cathode of the invention.
The production of a cathode of the invention comprises depositing a lead layer of reduced density from a lead salt-containing aqueous catholyte solution by electrochemical deposition onto an electrically conducting support. This cathode is connected as a cathode, and used in an electrolysis cell known per se.
Any cell known to a person skilled in the art is suitable as an electrolysis cell. A selection of frequently used cells is to be found in text books, for example in xe2x80x9cIndustrial Electrochemistryxe2x80x9d by Pletcher and Walsh, Chapman and Hall, London 1989, pp. 141-166.
Lead salts suitable for the lead salt-containing aqueous solution are all lead salts which contain lead in the divalent state, for example lead(II) carbonate, lead(II) chloride, lead(II) fluoride, lead(II) acetate, lead(II) formate, lead(II) oxalate, lead(II) nitrate and their basic salts, lead(II) sulfate and lead(II) oxide.
It is also possible to use a combination of metallic lead with an appropriate acid, from which a lead salt-containing solution is then formed.
A suitable solvent is water with the addition of mineral acids, preferably hydrochloric acid, sulfuric acid and phosphoric acid in a concentration between 0.5 M and 12 M, in the case of hydrochloric acid up to 8.5 M. Further possible additions are organic acids, for example acetic acid, formic acid, citric acid, lactic acid or their salts with ammonium, tetraalkylammonium, sodium or potassium ions, or complexing agents for divalent lead ions, for example EDTA or NTA in amounts of from 1 to 3 equivalents based on the amount of lead present.
The lead salt-containing solution may, where appropriate, additionally contain inorganic salts as conducting salts. Examples thereof are all alkali metal, alkaline earth metal and ammonium salts, for example sodium chloride, sodium sulfate, ammonium chloride, lithium perchlorate.
The lead salt-containing aqueous solution may additionally contain, where appropriate, water-miscible cosolvents such as ethanol, methanol, i-propanol, n-propanol, acetonitrile, ethylene glycol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, tetrahydrofuran, 1,4-dioxane, methyl acetate, ethyl acetate, dimethylformamide, sulfolane, ethylene carbonate, N,Nxe2x80x2-dimethylpropylene-urea, N,Nxe2x80x2-dimethylethyleneurea, N-methylpyrrolidone, tetramethylurea and hexamethylphosphortriamide in concentrations of from 0.1 to 80% by weight, preferably 1 to 50% by weight.
The lead ion concentration in the lead salt-containing aqueous solution is preferably between 1 ppm and 20 g/l, particularly preferably between 10 ppm and 10 g/l.
The pH of the electrolysis solution is preferably between 1 and 6, particularly preferably between 1 and 3.
The deposition of the lead layer is preferably achieved by applying a cathode potential of between xe2x88x920.1 V and the value at which evolution of hydrogen starts at the cathode. Current densities of from 0.1 A/m2 to 4000 AR/m2, preferably 10 to 3000 A/m2, are set in this case.
The temperature when carrying out the process is not critical. It can be chosen freely in wide ranges. A lower temperature limit is set by the freezing point of the catholyte solution, and an upper limit is set by the stability of the electrolysis cell. In the case of membrane cells, this is in particular the thermal stability of the membrane. In this case, the maximum temperature is limited to 60xc2x0 C.
The formation of the lead layer can also take place before or at the same time as the reduction of an organic compound.
The invention therefore also relates to a process for the electrochemical reduction of an organic compound, wherein, in place of a conventional cathode made of solid lead or a cathode with a solid lead coating, for example a lead film, there is the use of the cathode of the invention.
This process can be carried out analogously to known electrochemical processes for the reduction of such compounds using the cathode of the invention (review: M. M. Baizer in Organic Electrochemistry, published by Marcel Dekker, New York 1991, pp. 362 et seq.).
The compound to be reduced is preferably an organic sulfur compound, for example a disulfide compound, a sulfinic acid, a sulfoxide or a thioether, or an organic carbonyl compound, for example an aldehyde or ketone. Particularly preferred disulfide compounds are cystine, N-alkanoylcystine, homocystine and N-alkanoylhomocystine. The concentration of the compound to be reduced is from 0.01 M to 10 M, preferably 0.1 M to 5 M. The solvent preferably used is an aqueous solution of mineral acid, for example hydrochloric acid, sulfuric acid or phosphoric acid. Also the solvent preferably used is an aqueous solution of sodium or potassium hydroxide, sodium or potassium carbonate or acetate, ammonia, ammonium chloride or ammonium acetate in concentrations between 0.1 M and 12 M. The electrochemical reduction of the cystine and homocystine is preferably carried out in aqueous hydrochloric acid, sulfuric acid, sodium or potassium hydroxide solution or in ammoniacal solution. The concentration of cystine can be between 0.1 M and the saturation concentration. Saturated cystine or homocystine solutions are particularly preferred. The reduction of N-acetylcystine is preferably carried out at pH values between 5 and 13.
The electrolyses are carried out at temperatures between 0xc2x0 C. and 70xc2x0 C., preferably at temperatures between 10xc2x0 C. and 50xc2x0 C.
The current densities are 0.1 A/m2 to 20,000 A/m2, preferably 10 to 5000 A/m2.
All divided cells known to the person skilled in the art can be used to reduce organic sulfur and oxygen compounds (review, for example, in xe2x80x9cIndustrial Electrochemistryxe2x80x9d by Pletcher and Walsh, Chapman and Hall, London 1989, pp. 141-166).
It is possible with the cathode of the invention to produce cysteine hydrochloride from cystine in an electrolysis solution containing hydrochloric acid with the unusually high current density of 2000 A/m2 and with an area-time output of 7.3 kg of cysteine hydrochloride hydrate per hour and m of membrane area (see equation 1 in scheme 1). This area-time output is constant on operation for a period of up to at least 5 days. No lead is detectable, with a detection limit of 0.5 ppm, in the electrolysis solution.
A comparative experiment using a conventional cathode made from solid lead gave, at 2000 A/m2, an area-time output of only about 4 kg per hour and m2 of membrane area. Particularly at conversions of over 90% in the case of the conventional lead cathode with the high current densities there is a pronounced slowing-down of the reaction because of the evolution of hydrogen prevailing. This is likewise to be observed during the use of the tin cathode described in DE 1 024 518 (see comparative examples 7 and 8).
It is also possible with the cathode of the invention very easily to carry cut other reductions such as, for example, the homolysis or activated C-S bonds, the reduction of sulfinic acids or the reduction of keto groups.